The invention herein described relates generally to sensorless detection of a stall condition of a motor and more particularly to the sensorless stall detection for an open-loop step motor system, although the present invention may have other applications.
Motors, and particularly hybrid step motors, have been employed in several fields such as the disk drives for magnetic head positioning systems, drives for hydraulic or pneumatic valves, as well as numerous other applications. One of the primary advantages to using hybrid step motors in motion systems is that they are typically run in an open-loop fashion. However, excessive load torque can cause the rotor to lose synchronization with the commanded position. This is usually an unrecoverable error. In systems where it is critical to detect stall, an encoder is used as a feedback device only to ensure that the rotor is still turning. This encoder is an added system expense and lowers overall system reliability.
Therefore, a need exists in the motor art for a sensorless method of stall detection.
The present invention provides a system and method for sensorless detection of stall in an open loop motor. The system and method are characterized by the detection of at least one stall indicating harmonic in the spectrum of the commanded phase voltage of a motor. Accordingly, stall detection can be accomplished in an open-loop system without the need for an encoder.
According to one aspect of the present invention, a motor system comprises a motor and a detector which monitors at least one stall indicating harmonic associated with a motor spectrum.
In an embodiment, the stall indicating harmonic includes an even harmonic of the commanded phase voltage (or current) and, more particularly, the second harmonic of the commanded phase voltage.
In an embodiment, a filter is provided to extract the stall indicating harmonic component from the commanded phase voltage and a comparator compares the extracted harmonic (or harmonics) to a threshold value. If the threshold value is exceeded, a stall condition is indicated. Other functional criteria may be utilized as desired.
The motor can be a hybrid stepper having 2, 3, or 5 phases or a variable reluctance motor. The detector may be a digital detector, an analog detector or a hybrid detector.
According to another aspect of the present invention, a method of detecting a stall condition of a motor comprises monitoring at least one stall indicating harmonic associated with a motor spectrum and determining whether the motor has stalled as a function of the at least one stall indicating harmonic.
In one embodiment, the at least one stall indicating harmonic includes even harmonics of the commanded phase voltage and, more particularly the second harmonic.
The method may be applied, for example, to a hybrid step motor or variable reluctance motor. The hybrid step motor may have 2, 3, or 5 phases.
In an embodiment, the motor may be controlled in accordance with whether a stall condition has been determined. The motor may be driven by a driving voltage including a fundamental harmonic component. The driving voltage may include an odd harmonic greater than the fundamental harmonic. The monitoring or determining may include at least one digital processing step, at least one analog processing step or at least one hybrid processing step.
The foregoing and other features of the invention are herein fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention. These embodiments, are indicative, however, are but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.